Not applicable.
This invention relates to micro electro-mechanical systems (MEMS). More particularly, this invention pertains to low inertia microactuators used to operate a switch, valve, piston, or other mechanism at high rates.
High speed, high precision actuation devices are becoming required for a proliferating number of applications, in diverse fields. In industrial applications, very precise put-and-place actuators are required in printed circuit fabrication processes. Scientific applications may require sensors with very precise resolution, which implies fine motions. Deep space astronomical observations may use multifaceted mirrors, each facet independently controlled by a precision actuator. Digital projection cameras manipulate a plurality of reflectors, in order to cast an image onto a projection screen. Drug delivery systems in medical therapeutic treatment may require valves or pistons of high accuracy. Fault detection in vehicular safety systems require devices with precise, high speed motion. Computer disk drives require the alignment of the data heads over the sub-micron data track to a sub 0.1 um accuracy. In each case, the actuator drives the member of interest to a precise position or at a precise rate.
In general, the device itself (e.g. plunger, shutter, piston) may be of arbitrarily small size and low inertia. However, the driving member, i.e. the actuator, is not. The actuator must provide the force, throw (or range), and bandwidth to accommodate the application. Particularly in the case of high speed devices, stringent design criteria are set on the physical and mechanical properties that the actuator must possess. It should have low inertia and low power requirements. For low cost applications, it should also be mechanically simple. These considerations have led to the miniaturization of familiar electromechanical devices, using photolithographic processing rather than machining bulk components. Formation of sub-millimeter scale electromechanical systems is now well known in the art, as Micro Electromechanical Systems, or MEMS.
Among the simplest MEMS actuators that can be fabricated is the cantilevered beam, a device wherein a beam of substrate material is formed by patterning the dimensions of the beam and etching a void beneath it. This technique is described in examples xe2x80x9cMicrofabrication of cantilevers using sacrificial templates,xe2x80x9d U.S. Pat. No. 6,016,693 by Viani, et al., and xe2x80x9cHigh vertical aspect ratio thin film structures,xe2x80x9d U.S. Pat. No. 6,015,599 by Keller, et al.
The beam has a finite stiffness determined by its shape and mechanical properties, and can thereby be deflected by application of force. The amount of deflection through small angles varies linearly with the applied force, that is, the beam deflection can be characterized by a spring constant. In most cases, the force applied is electrostatic: The beam, suspended over the void and substrate, forms a parallel plate capacitor with the substrate being the opposing electrode. Actuation, or movement of the beam, results from the application of a differential charge, or voltage, between the beam the substrate.
The device to be actuated, for example a mirror, is then mounted upon the beam, and steered by the electrostatic force between the beam and the substrate.
Cantilevered actuators, while relatively simple in concept and construction, are also limited in performance. Deflection must be perpendicular to the plane of the substrate, as this plane defines the parallel plate capacitor. Additional beams, gears and bearings can translate this motion out-of-plane, as in Ho et al., in U.S. Pat. No. 5,629,918 (1997), xe2x80x9cElectromagnetically actuated micromachined flap.xe2x80x9d In this invention a flap, which is the moving member of the actuator, is coupled by one or more beams to a substrate and thereby cantilevered out of the plan of the substrate. While conceptually this invention allows larger motions in out-of-plane directions, the need for multiple beams and pivots seriously complicates the design and fabrication of the device, and deleteriously affects tolerances and rigidity.
Another difficulty with cantilevered actuators is that precise motion and high bandwidths require relatively stiff cantilevers. But since deflection is linearly proportional to the spring constant, a stiffer beam requires more force to achieve a certain throw. The tradeoff between stiffness, throw and bandwidth relegates cantilevers to a narrow range of applications. They are suitable for small ranges of motion, or in situations where large supply voltages are available.
Electrostatic forces are also relatively weak and provide actuation over small ranges, compared with, for example, magnetostatic forces. For this reason, magnetostatic devices are often preferred over electrostatic devices. Micromachined solenoidal magnetic actuators are known in the art, as micro-solenoid switches. Typically, a slug of magnetic material is affixed to a piston or plunger, and a coil is provided whose diameter is sufficient to admit the slug into its interior. The coil is then energized to repel or attract the slug, depending on the direction of current in the coil. The resulting linear mechanical motion is used to actuate various linear devices, such as opening and closing a switch or valve, or driving a piston.
An embodiment of a linear, solenoidal microactuator is found for example, in Guckel, et al., U.S. Pat. No. 5,644,177 (1997), xe2x80x9cMicromechanical magnetically actuated devices.xe2x80x9d The microactuator in this patent comprises a ferromagnetic mandrel around which a fine electrical wire conductor is wound, the mandrel further including pegs which locate and mate with corresponding receptacle holes in the stationary magnetic core.
Linear magnetic actuators are capable of higher forces and larger ranges of motion at lower driving voltages than cantilevered electrostatic actuators. They are therefore capable of actuating relatively large loads or operating against large spring constants. However, their throw is limited to the characteristic dimensions of the solenoid. Also, they operate against a spring force, required to return the moving member to the home position. This spring force requires more force or less throw, for a given energy density in the device. The spring also imparts a vibration to the device being actuated, and in general, the device is not functional until the vibration has ceased. This can add significant settling time to the switching speed.
A third design option is a rotary actuator. This device resembles a miniaturized electromagnetic motor, with a ferromagnetic material deposited on the substrate and wound with an electrical coil. Energizing of the coil induces magnetic flux in the permeable material. Generally the core is patterned with some arrangement of gaps, into each of which protrudes a driven member which interacts magnetostatically with the flux across the gap. A plurality of such elements, when driven in the proper sequence and timing, can produce a positive torque on a freely rotating member. A wide variety of designs for these magnetostatic micromotors can be found in the body of MEMS patents and publications, notably Garcia et al., U.S. Pat. No. 5,917,260 (1999) xe2x80x9cElectromechanical millimotor;xe2x80x9d xe2x80x9cSurface Micromachined Microengine,xe2x80x9d-E. J. Garcia, J. J. Sniegowski, Sensors and Actuators, A 48, pp. 203-214 (1995); and U.S. Pat. No. 5,631,514 xe2x80x9cMicrofabricated microengine for use as a mechanical drive and power source in the microdomain and fabrication process.xe2x80x9d
Notwithstanding the details of the various designs, the micromotors are conceptually similar to the familiar large scale rotor/stator electromagnetic motors.
Magnetostatic micromotors can be used as rotary actuators by mounting the device of interest onto the moving member, i.e. the rotor. This concept is clearly described in Mehregany, et al. in U.S. Pat. No. 6,029,337 (2000), xe2x80x9cMethods of fabricating micromotors with utilitarian features.xe2x80x9d This patent describes a micromotor fabricated using the same general principles as found in the MEMS art, but with additional material deposited on the rotor, constituting the utilitarian feature, such as fins, pump impellers, or optical elements. Energizing the stator induces rotary motion of the rotor bearing which rotates the affixed element into the desired position.
Micromotors overcome some of the limitations of cantilevers and solenoids, by having a large range of motion. However the requirement of a stator and freely rotating bearing constitute a substantially more complex device to make via MEMS processing. Further difficulties arise with rotor/stator actuators. As is well known with macroscopic motors, the initial direction of rotation is ambiguous: it may be either clockwise or counterclockwise. In addition, the rotation direction cannot be determined until sufficient motion has allowed sensing of the rotation. A correlated difficulty is that there is no defined xe2x80x9chomexe2x80x9d position of the rotor when the device is not energized. Again direct measurement means must be provided to determine the degree of rotation from a specific home point.
Therefore, a low inertia, high bandwidth, large throw, low power microactuator is a distinctly felt need in a wide variety of applications. However the prior art suffers from a number of disadvantages, including the following:
a) Cantilevered devices must be energized to maintain a given position against the restoring force of the spring constant, and therefore require constant power;
b) Cantilevers have limited throw, as the deflection is linearly related to the spring constant;
c) Cantilevered devices are generally electrostatic, further limiting their practical operating range;
d) Cantilevered devices which use additional beams or pivots to amplify or translate motion, suffer a loss of precision, repeatability and uniformity across devices, as well as complication of design and fabrication;
e) Solenoidal or in-line linear devices have a range of motion limited to the characteristic dimension of the coil;
f) Solenoidal devices operate against a fixed spring, and therefore dissipate power as does a cantilevered device;
g) Rotary devices are complex to fabricate because of the plurality of driving structures and requirement of a freely rotating bearing;
h) Rotary devices have an ambiguous direction of motion;
i) Rotary devices have an undefined home or dxc3xa9tente position.
j) Other known actuators provide for motion but not braking of that motion, leaving the device in vibration after actuation. This lengthens the total switching time, which includes actuation time plus sufficient settling time.
The present invention overcomes the limitations of the prior art for low inertia, high bandwidth actuators. The invention is a hybrid device, incorporating one or more features of each of the basic types of microactuators: cantilevered, solenoidal, and rotary. The actuator comprises a magnetic core with a gap affixed to the substrate, and wound with an electrical coil, as in a micromotor. Energizing of the coil induces a magnetic flux through the core material and across the gap in the core. However the driven member is not a piston, plunger, or rotor, but rather a hinge-mounted member, which pivots about a stationary point. The driven member includes a tab of magnetic material, which interacts with the core gap field, to impel motion of the member about the pivot point.
The pivoting motion is enabled by a flexible hinge structure, which connects the driven member to the stationary pivot point. Use of a hinge rather than a true bearing, results in a cantilever-like device with a range of motion substantially less than 360 degrees. This range is ample for most applications, which require simply an xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d position. Avoidance of a bearing structure significantly simplifies the design and fabrication of the device. The limited range of motion allows for the use of a single toroidal driving structure rather than a plurality of synchronized drivers as in a true rotary actuator.
Another feature is the inclusion of a dxc3xa9tente latching spring system, with two or more stable positions, such that the pivoting member may be latched in any one of the stable positions upon power-down. It thereby avoids the difficulties encountered with true rotor/stator micromotors, wherein no unique home position can generally be defined.
Lastly, the invention includes an auxiliary magnetic circuit for unambiguously sensing the orientation of the pivoting member. The sensing circuit can distinguish between the plurality of stable positions, and can be equipped with a feedback loop to fine tune the driving circuit for optimum performance. The sensing apparatus and feedback loop make possible braking action, or even full servo control, to minimize vibration and settling time of the pivoting member.
As a result of its novel design, this invention achieves the following objects and advantages over the prior art:
a) To provide low inertia, high bandwidth actuation;
b) To provide a large range of motion in the actuation;
c) To provide an actuator with an unambiguous direction of motion;
d) To provide an actuator whose position can be determined by a simple, on-board measurement circuit; and
e) To provide a low loss actuator, which can maintain one of a plurality of stable positions in the quiescent (unenergized) state.
f) To improve switching times by applying a braking force or full servo control of the pivoting motion.
A further object of this invention is to use variations of known micromotor fabrication techniques, in order to avoid further process invention.
A further object of the invention is to achieve the design using, where possible, known MEMS microcomponents, subassemblies, and electronic circuitry.
Still a further object of the invention is a design sufficiently simple, that it can be practiced in various thin-film fabrication facilities, which use slightly different tools and procedures, to achieve a reproducible, uniform and predictable product, without undue experimentation or adaptation of the equipment.
Still further objects and advantages will become apparent to those of ordinary skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description and the specific examples being presented, while indicating certain embodiments of the invention, are provided for illustration purposes only. Various changes and modifications within the scope and spirit of the invention will become apparent to those of ordinary skill in the art from the detailed description of the invention and the claims that follow.